This invention relates to a method and apparatus for printing an image and, more specifically, relates to a method for expressing gradation in thermal transfer printing and an apparatus using the method.
As an example of a thermal printing apparatus, there is known a color copying machine of the thermal transfer type which reproduces a color image by using thermal transfer ink ribbons having a plurality of colorants. In a conventional apparatus of this kind, the ink ribbons are positioned opposite a copy paper, and are heated selectively by a thermal printing head having a plurality of heating elements. In operation, the heating elements are energized according to the image to be printed, whereby the colorants are transferred to the copy paper to form the image. Examples of such prior art machines are shown, for example, in U.S. Pat. Nos. 4,067,017; 4,378,566; and 4,427,985, and European Patent Application No. 0,050,481.
In the printing apparatus mentioned above, however, as all the colorants have the same melting point, the apparatus may not be utilized to obtain an intermediate tone or sufficient gradation. It is, therefore, known to achieve gradations in the hard copy by controlling the image density by varying the driving time period of the heating elements, or by controlling the image density of printed dots by varying the number of driven heating elements assigned to form a picture element. However, the relationship between the image density of the printed image and the driving time period of the heating elements is not linear. Thus, if a smooth gradation is to be achieved by varying the driving time period, complicated conditions must be met. Further, one picture element is defined by a plurality of recording dots. Therefore, the resolution of the printed image deteriorates when a smooth gradation is obtained by varying the number of driven heating elements. To reduce this deterioration, the number of heating elements defining one picture element could be increased, if desired. However, the density of heating elements should be limited to 16 elements/mm due to manufacturing considerations. Therefore, a need has arisen for a method and apparatus for achieving a smooth gradation without producing a deterioration in resolution by using a low density arrangement of heating elements.